Hardenability
It is the ease or ability of the steel to form Martensite on quenching. If the slope of CCR (Critical Cooling Rate – Cooling Curve which just touches the Nose of TTT curve) is very high, the cooling rate has to be very high and hence the hardenability is low (as Martensite will not form unless the steel is quenched rapidly)
Effect of Carbon on Hardenability
(A) Hypoeutectoid Steel
The TTT Curve shifts to the left compared to Eutectoid steel as the Pro-Eutectoid Ferrite provides sites for Pearlite Nucleation and promotes Pearlite Formation
Hence as the Carbon content increases in the Hypo range, the amount of pro-eutectoid ferrite (can be checked from Lever Rule) decreases and hence the Hardenability increases (as Pearlite formation is allayed) up to 0.8% wt. C (Eutectoid composition)
The same is reflected in the TTT Curve that as the Carbon content increases in the Hypo range, the TTT curve shifts to the right up to the 0.8% at. C and the approaches the TTT curve of Eutectoid Steel which is shown below
(B) Hypereutectoid Steel
In this case also, the C-Curve shifts to the left compared to Eutectoid steel as the Pro-Eutectoid Cementite formed acts as nucleating sites for Pearlite formation (Please note that Pearlite forms by Nucleation of Cementite, hence pre-existing Cementite eases the Pearlite formation)
Hence the as carbon content increases beyond 0.8% wt. C, the amount of pro-eutectoid cementite increases and hence hardenability decreases
